PIA11600: Jagged Shadows

The shadows cast by Daphnis’ attendant edge waves create a dark, jagged
pattern on the A ring in this image taken as Saturn approached its August
2009 equinox.

Daphnis (8 kilometers, or 5 miles across) is a bright dot in the Keeler
Gap of the A ring just below the center of the image. The moon has an
inclined orbit, and its gravitational pull perturbs the orbits of the
particles forming the Keeler Gap’s edges and sculpts both edges into waves
having both horizontal (radial) and out-of-plane components. Material on
the inner edge of the gap orbits faster than the moon so that the waves
there lead the moon in its orbit. Material on the outer edge moves slower
than the moon, so waves there trail the moon. See PIA11656 to learn more
about this process.

The novel illumination geometry that accompanies equinox lowers the sun’s
angle to the ringplane, significantly darkens the rings, and causes
out-of-plane structures to look anomalously bright and cast shadows across
the rings. These scenes are possible only during the few months before and
after Saturn’s equinox, which occurs only once in about 15 Earth years.
Before and after equinox, Cassini’s cameras have spotted not only the
predictable shadows of some of Saturn’s moons (see PIA11657), but also the
shadows of newly revealed vertical structures in the rings themselves (see
PIA11665).

This view looks toward the northern, unilluminated side of the rings from
about 36 degrees above the ringplane.

The image was taken in visible light with the Cassini spacecraft
narrow-angle camera on July 28, 2009. The view was obtained at a distance
of approximately 1 million kilometers (621,000 miles) from Daphnis. Image
scale is 6 kilometers (4 miles) per pixel.

The Cassini-Huygens mission is a cooperative project of NASA, the European
Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory,
a division of the California Institute of Technology in Pasadena, manages
the mission for NASA’s Science Mission Directorate, Washington, D.C. The
Cassini orbiter and its two onboard cameras were designed, developed and
assembled at JPL. The imaging operations center is based at the Space
Science Institute in Boulder, Colo.